Dielectrically-loaded quadrifilar helical antennas are disclosed in British Patent Applications Nos. 2292638A, 2310543A, and 2367429A and International Application No. WO2006/136809, the latter being related to U.S. patent application Ser. No. 11/472,586 filed Jun. 21, 2006. Such antennas are intended mainly for receiving circularly polarised signals from a global navigation satellite system (GNSS), e.g. from the satellites of the Global Positioning System (GPS) satellite constellation for position fixing and navigation purposes. GPS in the L1 band and the corresponding Galileo service are narrowband services. There are other satellite-based services requiring receiving or transmitting apparatus of greater fractional bandwidth than that available from the prior antennas. One antenna offering increased bandwidth is that disclosed in British Patent Application No. 2424521A. A dual band dielectrically loaded antenna system is disclosed in British Patent Application No. 2311675A. An antenna capable of receiving circularly polarised signals and having a resonant ring conductor is disclosed in European Patent Application No. 1147571A.
Related antennas are disclosed in British Patent Application No. 2445478A and related U.S. patent application Ser. No. 11/970,740 filed Jan. 8, 2008. These applications disclose hexafilar and octafilar antennas offering greater bandwidth and/or higher gain than a comparable quadrifilar antenna. A high-impedance quadrifilar antenna is disclosed in British Patent Application No. 2444388A and related U.S. patent application Ser. No. 11/998,471 filed Nov. 28, 2007.
The entire disclosures of the above applications are incorporated in that of the present application as filed by reference.
It is an object of the present invention to provide an antenna capable of receiving circularly polarised radiation at first and second resonant frequencies.